The present invention relates to an antifriction bearing for use in an environment involving vibration or impact, and also to an alternator incorporating the bearing for use in vehicles.
The bearing rings and rolling members of the antifriction bearing (hereinafter referred to simply as the xe2x80x9cbearingxe2x80x9d) are generally subjected to a cyclic high-shear stress due to a rolling motion, so that they are given high Rockwell hardness of HRC 58 to 64 by hardening and tempering so as to retain increased strength against rolling fatigue. It has been reported that there is the following relationship between the hardness and the rolling fatigue life. The life greatly shortens as the hardness decreases.
LH=ƒHp.L
wherein
LH: the life when the hardness varies
fH: the hardness factor
p: a constant (3 for ball bearings, or 10/3 for roller bearings)
L: the life of the standard bearing Further,
fH=(HV/750)2 
wherein HV is Vickers hardness
Recently, however bearings are used for applications wherein the intended rolling fatigue life is not available merely by assuring the hardness.
With common bearings, the shearing stress due to the contact between the bearing rings and the rolling members develops a crack from an inclusion or the like, and the crack grows to cause flaking. In the case where the inner ring is rotated, such flaking occurs predominantly in the rotating ring, i.e., the inner ring. Conversely, in the case of bearings which are used in an environment involving vibration or impact, the vibration or impact causes many minute cracks or changes in the structure immediately under the raceway of the outer ring which is fixed, consequently giving rise to flaking within a very short period of time to render the bearing unserviceable.
This phenomenon appears attributable to the following reason. The vibration or impact deforms the raceway and becomes more pronounced, consequently causing greater microscropic strain in the ring under the raceway.
The rolling fatigue life can be lengthened most easily by increasing the size of the bearing to give an increased load capacity. This achieves an advantage since a reduced stress value will then result when the same load is applied. In actual use, however, the vibration or impact load readily varies with the structure around the bearing and mounting and operation conditions, and it is impossible to meet the requirement of decreasing the size and weight, so that the increase in the size of the bearing is not a satisfactory solution.
High-carbon chrominum steel (such as JIS SUJ2 or SAE 52100) as adjusted to the hardness of HRC 58 to 64 by the usual hardening and tempering treatments as stated above is conventionally used for the inner and ouster rings of the bearings for alternators for vehicles.
In recent years, however, alternators are required to have a smaller size, reduced weight and higher output to meet the need to decrease the fuel cost of vehicles and increase various electrical loads thereof. To fulfill this requirement, it has become practice to use a greater pulley ratio and to rotate the alternator at a high speed. Accordingly, the maximum speed of rotation is in excess of 12000 r.p.m.
The problems involved in the high-speed rotation include the slippage of the belt in connection with the external arrangement of the alternator. This problem has been overcome by using a larger number of belts under increased tension. On the other hand, the problem associated with the internal arrangement of the alternator is the need to render the bearing resistant to the high-speed rotation and to the high tension involved. More specifically, because the heat of agitation due to the high-speed rotation and the increased frictional heat due to high tension shorten the life of the grease used, the bearing must be adapted to overcome this problem. Furthermore, the bearing must be rotatable at a high speed without marked vibration that would result from the deformation of the raceway due to high tension. Generally, the bearing is rotatable at a high speed satisfactorily when reduced in size, since the side reduction is effective for decreasing the amount of heat generation.
Nevertheless, in the case where the bearing is subjected to high tension as in the alternator, a reduced size leads to a decreased load capacity to entail a shortened fatigue life, so that the bearings in use are at least about 32 mm in outside diameter if smallest.
Briefly, in assuring high-speed rotation under increased tension as required for reducing the size and weight of the alternator and increasing the output thereof, it is necessary to solve the conflicting problems of inhibiting heat generation and taking a countermeasure against the increased load while diminishing the vibration, whereas difficulties are encountered with the convention alternator described in overcoming these problems.
An object of the present invention is to a bearing which is adapted to have a prolonged life in an environment of vibration or impact without increasing the size of the bearing.
Another object of the invention is to provide an alternator which is rotatable at an increased speed and is nevertheless operable under the resulting increased tension, as required for reducing the size and weight of the alternator and increasing the output thereof.
As already described, we have found that the early flaking of the outer ring of the bearing in use under vibration or impact is attributable to many cracks or changes in structure which occur immediately under the raceway under the dynamic action of excessive stress due to the severe load of the vibration or impact, and carried out repeated testing and research with attention directed to the heat treatment of the outer ring to give the ring resistance to cracks or changes in structure, whereby the present invention has been accomplished.
Stated more specifically, the present invention provides a bearing wherein the ring to be fixed comprises a steel up to about 10% in the amount of residual austenite.
The amount of residual austenite in the fixed ring should be up to about 10% for the following reason. The usual hardening-tempering treatment of steel permits about 11 to about 14% of austenite to remain on the average, and it is said that a somewhat higher content of residual austenite leads to an improved rolling fatigue life.
For example, C. Razim carburized steels such as 14NiCr14 (0.14% C, 0.46% Mn, 0.78% Cr and 3.67% Ni), 16MnCr5 and 20MoCr4 and tested the steels for fatigue by contact with a roller with the following conclusions (see C. Razim, Hxc3xa4rterei Technische Mitteilungen, 22(1967), Heft 4, S. 32).
(1) The surface of the steel in contact with the roller underwent plastic deformation due to load stress. The width of contact therefore increased to result in a lower contact surface pressure consequently improving the pitting life.
(2) Rotation bending fatigue test revealed that the specimen containing 30 to 50% YR (residual austenite) was about 2 times the specimen of pure martensite in fatigue strength improvement.
(3) 14NiCr14 specimen with 50% YR was HV 550 in hardness. The testing changed the surface hardness to HV 950.
(4) It was not apparent whether the testing converted the YR to martensite. After the testing, a carbide was observed in the structure microscopically.
J.P. Sheahan et al. carburized SAE 8620 steel under varying conditions and subjected the steel to a pitting test with a roller to ford that the specimens with a higher YR content were longer in pitting life than those with a lesser content. (see J. P. Sheahan and M. A. H. Howes, SAE 720268). A plastic flow and work hardening are suggested as the reason. O. W. Mcmullan shares the same concept as above, stating that the presence of YR is likely to mitigate the load stress (see O. W. Mcmullan, Metal Progress (1962) April, p. 67).
According to R. A. Wilde, up to 10% of YR is not appropriate because of excessive hardness. He states that the presence of a proper amount of YR, which is optimally 10 to 25%, is useful for mitigating the load stress (see R. A. Wilde, Research Center Eaton and Towne Inc., (1967), Oct.).
Yajima et al., conducted a rolling fatigue test using bearing steel, with the result that the pitting life improved with increasing YR content (Yajima et al., The Japan Institute of Metals, Symposium, 1972)
Since a detailed examination of the portion immediately below the point of contact between the specimen and the steel ball indicated that the test increased the hardness from HV 750 to about HV 1000 and that the X-ray diffraction line due to austenite almost disappeared, they postulated that the result was due to the overall effect of ausforming and stained induced transformation. Like Yajima et al., Okamoto et al. carried out a rolling fatigue test using bearing steel to investigate the influence of YR on pitting life (see Okamoto et al., Seitetsu Kenkyu (Research on Iron Making), 1973, No. 277, p. 82). They directed attention to the fact that a specimen, containing YR, having a softer surface than the one almost free from YR, underwent plastic deformation at the surface to exhibit a reduction in the substantial surface pressure, when the specimens were subjected to the same load, and compared the tested specimens with the surface pressure corrected to find that the specimens having a higher YR content exhibited a longer pitting life than those with a lower YR content, as demonstrated by Yajima et al. The reasons given for the result achieved by the specimens with a higher YR are the function of a strain concentrator which repeatedly absorbs stress to prevent the occurrence and development of cracks, and hardening due to convertion to martensite due to work as shown in FIG. 1.
However, when a large amount of residual austenite is present, the structure of steel is unstable under vibration or impact, exhibits lower strength as shown in FIG. 4 and is susceptible to plastic deformation as already stated, with the result that the raceway deforms to permit further pronounced vibration or impact. The rolling frictional force also increases as shown in FIG. 5 to result in increased stress and to produce increased strain in the ring under the raceway. Alternatively, the structure is prone to a change due to strain induced transformation, and the resulting martensite structure, which is not tempered, is brittle.
Consequently, if the amount of residual austenite exceeds about 10%, the ring becomes susceptible to a local change in structure or to cracking due to vibration or impact. The amount of residual austenite is herein limited for the fixed ring based on the fording that the bearing life is substantially dependent on the damage to the fixed ring because the fixed ring, the loading region of which is more definite, is subject to the influence of vibration or impact more greatly. Preferably, the residual austenite content is up to 6%.
The residual austenite content can be reduced by conducting a sub-zero treatment between hardening and tempering. The sub-zero treatment converts austenite to martensite to decrease the residual austenite content.
The residual austenite content can be reduced also by adjusting at least one of the hardening heating temperature, hardening cooling rate and tempering temperature. For example, although tempering for usual bearings is conducted at 150xc2x0 to 200xc2x0 C. to obtain hardness of HRC 58 to 64, the tempering treatment, when carried out at a higher temperature of 250xc2x0 to 380xc2x0 C. reduces the residual austenite content, whereby the possible change in structure or cracking can be precluded.
The high-temperature tempering at 250xc2x0 to 380xc2x0 C. gives hardness of HRC 52 to 57 and is therefore likely to shorten the usual flaking life under the common operating conditions, whereas in the case where a vibration or impact load is involved, the structural change or cracking can then be prevented as stated above to result in a greatly lengthened service life.
The present treatment is conducted on the fixed ring, while the rotatable ring is treated in the same manner as usual bearings. By following this procedure, bearings are produced which, under the common operating conditions, show no problems.
Thus, the fixed ring of the bearing of the present invention is made of a steel having a residual austenite content of up to about 10%. Accordingly, even if subjected to vibration or impact, the ring is less prone to deformation at its raceway, remains stable in structure and is resistant to structural changes or cracking, with the result that the bearing is usable for a prolonged period of time in an environment involving vibration or impact without the necessity of being made larger in size.
The present invention further provides an alternator wherein the rotary shaft of a rotor is rotatably supported by a pair of bearings on a frame having a stator, and a drive pulley is mounted on one end of the rotary shaft projecting outward from the frame. The alternator is characterized in that the outer ring of at least the bearing toward the pulley comprises a steel up to about 10% in the amount of residual austenite, so as to preclude the marked vibration to be produced during high-speed rotation by the deformation of the raceway due to high tension.
With the alternator of the present invention, the bearing outer ring is reduced in residual austenite content as already described and is thereby prevented from plastic deformation at the raceway, whereby the raceway is prevented from caving in unevenly to assure diminished vibration, a reduced frictional force and inhibited heat evolution. This realizes a high-speed operation under increased tension, making it possible to reduce the size and weight of the alternator and increase the output thereof.